They aren’t useless. Eventually you have to rebuild a society, and to make a society at some point you have to stop killing everybody. That’s always been the long game concept of TWD, it’s the story of going all the way THROUGH zompoc to having dealt with it and having a functioning society again.
No, you have to KEEP killing the right people. You preserve the society you've built by eliminating the threats to that society. It started as the zombies. Then it was the Terminites. Then it was the Goobner. Now it's Negan. It will always be somebody.
But you don't have to agonize over it every freaking episode!